A fuel cell has been proposed as a clean, efficient, and environmentally responsible energy source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for the traditional internal-combustion engine used in modern vehicles. One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to a vehicle.
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in the fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte, and thus, are directed through a load to perform work before being sent to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. Not all of the hydrogen is consumed by the stack, and some of the hydrogen is output as an anode exhaust gas that may include water and nitrogen.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane, for example. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane define a membrane electrode assembly (MEA).
Several individual fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells. The fuel cell stack typically includes fuel cell subsystems and related devices that aid in the preconditioning and operation of the fuel cell stack. As nonlimiting examples, the fuel cell subsystems and related devices housed within the main body can include end plates, fluid passages, e.g. hydrogen fuel and oxidant (O2/air) passages, coolant pumps, recirculation pumps, drainage valves, fans, compressors, valves, electrical connections, reformers, humidifiers, water vapor transfer units, heat exchangers, and related instrumentation.
Liquid water present in the fuel cell stack and the fuel cell subsystems may prevent optimal operation of the fuel cell stack. Liquid water may block gas flow within the fuel cell stack and the fuel cell subsystems and may freeze when the fuel cell stack is not operating. A portion of the anode exhaust gas may be recycled to maintain an anode stoichiometry without the use of excess hydrogen. When cold hydrogen is injected into a desired anode reactant recycler in fluid communication with the anode, water vapor present in the exhaust gas condenses and is separated from the exhaust. Ice may prevent the operation of a combined bleed and drain valve used to remove the water and excess nitrogen from the fuel cell stack, increasing a startup time of the fuel cell stack. The combined bleed and drain valve minimizes complexity and is typically located adjacent a water collecting portion of the fuel cell subsystem the combined bleed and drain valve is incorporated in. Alternately, separate valves (a bleed valve and a drain valve) may be used to perform bleed and drain functions in the fuel cell subsystem.
Excess nitrogen may be present in the anode of the fuel cell stack and the fuel cell subsystems as a result of extended periods of non-operation of the fuel cell stack, or as a result of bleed through from the cathode. Nitrogen present within the fuel cell stack results in a poor performance of the fuel cell stack. Accordingly, the excess nitrogen must be bled from the system. The bleed valve may also be used to remove the excess nitrogen. Ice may prevent the operation of the bleed valve used to remove the nitrogen from the fuel cell stack, preventing optimal operation of the fuel cell stack.
The cathode exhaust gas may be used to humidify oxygen or air entering the cathode using a water vapor transfer unit (WVT). Liquid water present in the cathode exhaust gas indicates the cathode is over humidified. When the WVT is exposed to liquid water, water in excess of the desired amount may be reintroduced into the cathode. To purge water from the cathode, a bypass valve may be used to direct the cathode exhaust gas away from the WVT. A bypass valve system is typically bulky and generally includes an actuator and a sensor.
Atmospheric air may be compressed and cooled before entering the WVT and cathode. Humidified air leaving the WVT during a cold start may condense and accumulate prior to entering the cathode. It is desirable to remove the condensate prior to the air entering the cathode. Removal of the condensate minimizes the startup time of the fuel cell stack in cold weather. The condensate is typically drained using an intermittently operated draining system. The draining system may take up considerable space and include a collection point, a drain valve, and a condensate level sensor.
It would be desirable to produce a passive water drain for a fuel cell stack that minimizes a complexity of an anode reactant recycler, eliminates the need for bypass valve systems used to remove water from the cathode exhaust, and eliminates the need for condensate draining systems used for compressed air entering the cathode.